Segmentation fault accessing 2D array

Question

I have a 2D array representing a sudoku grid defined like this:

int** sudoku = (int**)malloc(sizeof(int*) * 9);
for(i = 0; i < 9; i++)
    sudoku[i] = (int*)malloc(sizeof(int) * 9);

I've run it through a function that iterates through every element and prints, which works fine (displays 81 zeros). But then, I hand it to another function that reads a grid from a file into this array. Here's what it looks like (with a bunch of printf statements I'm using for debugging omitted).

void readSudokuFile(char* filename, int*** grid) {
    FILE* file;
    int i, j, curr;

    file = fopen(filename, "r");
    for(i = 0; i < 9; i++) {
        for(j = 0; j < 9; j++) {
            fscanf(file, "%d", &curr);
            *grid[i][j] = curr;
        }
    }
    fclose(file);
}

When the function is running, it appears to read find for the first row, but when it gets to the second row and tries to insert a value into sudoku[1][0], I get a seg fault. This is what the output looks like with my printfs in:

Reading line 0...
    Reading col 0... got 6
    Reading col 1... got 2
    Reading col 2... got 4
    Reading col 3... got 5
    Reading col 4... got 3
    Reading col 5... got 9
    Reading col 6... got 1
    Reading col 7... got 8
    Reading col 8... got 7
Reading line 1...
    Reading col 0... got 5
Segmentation fault(core dumped)

This is the file I'm trying to read in:

6 2 4 5 3 9 1 8 7
5 1 9 7 2 8 6 3 4
8 3 7 6 1 4 2 9 5
1 4 3 8 6 5 7 2 9
9 5 8 2 4 7 3 6 1
7 6 2 3 9 1 4 5 8
3 7 1 9 5 6 8 4 2
4 9 6 1 8 2 5 7 3
2 8 5 4 7 3 9 1 6

I'm compiling using gcc with -Wall and -pedantic, and am getting no compiler warnings.

I've googled around for a few hours to no avail, I'm not exactly sure what's going wrong here. Any help would be greatly appreciated.

no need to cast your malloc return values. How are you calling this readSudokuFile function? Run valgrind on your code. — t0mm13b
– t0mm13b, Commented May 7, 2017 at 21:12
Change *grid[i][j] = curr; to (*grid)[i][j] = curr;. But really, int sudoku[9][9] would have made things a lot easier. And there's no need to pass a pointer to grid to this function, unless it's liable to reallocate the array at some point. — r3mainer
– r3mainer, Commented May 7, 2017 at 21:13
Completely forgot about valgrind! I'll have a look with it. I call the function with readSudokuFile(argv[1], &sudoku) — rjt197197
– rjt197197, Commented May 7, 2017 at 21:15
There is no 2D array in the code shown and nothing whch can point to one. A pointer is not an array. And being a 3-star (***) C programmer is not a compliment! — too honest for this site
– too honest for this site, Commented May 7, 2017 at 21:18
@rjt197197: There is no problem using an array. If you had problems, you did something wrong. This typically signals one has not understood the concepts correctly. And the ideomatic way to use a dynamically allocated 2D array is to use a pointer to a 1D array. Either way, int ** is the wrong type. int *** is complete nonsense. Functions can return a result! — too honest for this site
– too honest for this site, Commented May 7, 2017 at 21:23

Anders Cedronius · Accepted Answer · 2017-05-07 21:47:51Z

2

To avoid pointer bugs you just ran into I suggest to simplify your dynamic 2D-array allocation like this ->

int (*array) [Y] = malloc(sizeof(int[X][Y]));

Access your array like this ->

int g=array[0][0];

And set like this ->

array[0][0]=0;

That will simplify your solution a lot and you also get a continuous memory block containing your 2D-array as a bonus. That will also simplify writing and reading files as you do not need to traverse each element if you do not necessarily have to do that for other reasons.

/A

edited May 7, 2017 at 21:47

answered May 7, 2017 at 21:39

Anders Cedronius

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Comments

Tu.Ma. · Accepted Answer · 2017-05-07 21:16:40Z

1

Try modify your function like this:

        void readSudokuFile(char* filename, int** grid) {
   // ...
                grid[i][j] = curr;
    }

and invoke it like this:

readSudokuFile(filename, sudoku)

answered May 7, 2017 at 21:16

Tu.Ma.

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